A. Technical Field
The present invention relates to a water-absorbing agent and a production process therefor, and in more detail, relates to: a water-absorbing agent, which 1) displays very high absorption capacity even under an increased pressure, 2) displays only a small difference between swelling capacities at positions in a layer, 3) displays uniform swelling action even when the density of a water-absorbent resin is high and even when a swollen gel of the resin is present in the form of the layer, and 4) is favorably used for sanitary materials such as disposable diapers, sanitary napkins, and incontinence pads; and a production process to provide the water-absorbing agent.
B. Background Art
In recent years, water-absorbent resins have been widely used in the manufacture of sanitary materials, such as disposable diapers, sanitary napkins, and incontinence pads, in order to allow the water-absorbent resins to absorb humors.
Known examples of water-absorbent resins are as follows: crosslinked products of partially neutralized polyacrylic acid; hydrolyzed products of starch-acrylonitrile graft polymers; neutralized products of starch-acrylic acid graft polymers; saponified products of vinyl acetate-acrylic ester copolymers; crosslinked matters of carboxymethylcellulose; hydrolyzed products of acrylonitrile copolymers or of acrylamide copolymers, or crosslinked products of these hydrolyzed products; crosslinked products of cationic monomers; crosslinked isobutylene-maleic acid copolymers; and crosslinked products of 2-acrylamido-2-methylpropanesulfonic acid with acrylic acid.
It is conventionally requested that the above-mentioned water-absorbent resins have the following properties: when contacting aqueous liquids such as humors, they display high absorption capacity, excellent absorption speed, liquid-permeability, gel strength of swollen gel, and a suction force, which draws water from base materials containing aqueous liquids, and so on. However, relationships between these properties do not necessarily display positive correlations. For example, the higher the absorption capacity, the lower some other properties such as liquid-permeability, gel strength, and absorption speed. Thus, for improving the water-absorption properties of a water-absorbent resin for a good balance among the properties, various methods have been proposed so far in this art where the neighborhood of the surface of the water-absorbent resin is crosslinked. For example, methods are known in each of which the following are used as crosslinking agents: polyhydric alcohols; polyglycidyl compounds, polyaziridine compounds, polyamine compounds, or polyisocyanate compounds, glyoxal; polyvalent metals, silane coupling agents; combinations of epoxy compounds with hydroxy compounds; alkylene carbonates.
In addition, methods are also known in which when a crosslinking agent is added, the following are allowed to be present: inert inorganic powders; dihydric alcohols; ether compounds; water-soluble polymers; alkylene oxide adducts of monohydric alcohols, or organic acid salts, or lactams, as an attempt to more uniformly disperse the crosslinking agent onto the surface of a water-absorbent resin and to carry out uniform surface-crosslinking in a crosslinking reaction.
The balance among the properties of a water-absorbent resin may be improved by the above-mentioned methods, but it is still difficult to say that the resultant balance is sufficient. As to the above-mentioned conventional water-absorbent resins as used for absorbent material in sanitary articles, the following are, for example, known: an water-absorbent sanitary article containing a water-absorbent resin of which the amount of the water absorption, especially that under an increased pressure, and the gel fracture strength are specified (Japanese Patent Application Publication (Kokai) No. 63-99861); a disposable diaper containing a water-absorbent resin of which the amount of the water absorption or the water absorption speed under an increased pressure is specified Japanese Patent Application Publication (Kokai) No. 2-34167); an absorbing agent containing a water-absorbent resin of which the amount of the water absorption under an increased pressure and the particle diameter under an increased pressure are specified (European Patent No. 339461); a water-absorbent resin containing a water-absorbent resin of which the water absorption speed or the amount of the water absorption under an increased pressure in a short time is specified (European Patent No. 443627). However, further enhancement of the quality is demanded. Under current circumstances, especially considering necessary properties of water-absorbent resins as used for absorbent material in recently trendy sanitary articles that use a large amount of water-absorbent resin and are thinned, properties resultant from the above-mentioned conventional methods have not yet attained a sufficient level.
That is to say, "an absorption capacity under a high load (e.g. 50 g/cm.sup.2) (which may hereinafter be referred to as absorption capacity under an increased pressure) is so excellent that sufficient absorbency can be displayed even if a heavier load is applied during the attachment" This is one of the necessary properties of water-absorbent resins as used for thin type absorbent material containing water-absorbent resins of high concentration. This necessary property can be relatively enhanced by the above-mentioned surface crosslinking methods, but sufficient absorbency can be displayed with regard to the entirety of the absorbent material.